1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile apparatus and the like, and to an image receiving sheet to be applied to the foregoing apparatus. More particularly, the present invention relates to an image forming apparatus capable of outputting a multi-color image and an image receiving sheet to be applied to the foregoing apparatus.
2. Description of the Prior Art
In recent years, a high quality color image has been required to perform a presentation or the like. Also research and development of the electrophotography has a requirement for improving the quality of the image including the color reproducibility and image density. In order to improve the saturation, image density and luster of the color image, an image receiving sheet, such as glossy paper, can be available as exclusive paper. The image receiving sheet is structured to embed toner into a resin layer on the sheet in order to prevent deformation and shift of dots when toner is fixed with heat and attain luster of the surface of the image. Since the image receiving sheet is required to have luster, light resistance and water resistance equivalent to the silver salt photography, toner must be deeply embedded into the resin layer by fixing and smoothness of the surface of the image must be realized. An image receiving sheet of a type to embed the toner uses a transparent sheet as the base thereof so as to be applied as a sheet for an over head projector (OHP). If the image receiving sheet is used as the OHP sheet, the difference in the smoothness of the surface determines the color development characteristic of the projected image. Accordingly, the image receiving sheet for electrophotography must have smoothness on the surface of the fixed image and therefore embedding of the toner into the resin layer is a critical factor.
To satisfy the above-mentioned requirements, Japanese Patent Publication No. Hei. 4-125567 has a structure in which an image receiving layer is formed which contains thermoplastic resin having a softening point lower than that of the color toner and a print in which the toner has been embedded in the image receiving layer and thus irregularity is prevented is obtained so as to solve the above-mentioned problem.
If a resin layer having a low softening point as disclosed in Japanese Patent Publication No. Hei. 4-125567 is applied to the surface of the image receiving sheet, the weak coagulation force of the melted resin results in that offset of the toner layer and the image receiving layer to the fixing roller easily takes place. When the image is stored, there arises a problem of fusion of the image receiving sheet due to blocking or the like.
By the way, the foregoing suggestion for forming the image receiving layer on the surface of the base sheet has been performed to be adaptable to an image receiving sheet for forming a monochrome image and an image forming apparatus arranged to use the foregoing image receiving sheet. An object of the foregoing suggestion is to improve the fixing characteristic in order to realize strength sufficient to prevent separation of toner from the image receiving sheet and to improve the conveyance easiness to prevent jamming of the image receiving sheet.
However, the image forming apparatus for outputting a color image and the image receiving sheet to be adapted to the foregoing apparatus must form toner images fixed on the image receiving sheet and having excellent color development characteristic and transparency in order to obtain a high quality color image in a manner different from the image forming apparatus for outputting a monochrome image.
To obtain an image having excellent color development characteristic and transparency, it is an important fact that the fixed toner image does not scatter light.
To prevent light scattering caused by the fixed image, the surface of the fixed image must satisfactorily be smoothed and the fixed image must be free from generation of an interface between toner particles. To realize his, a method has generally been employed in which the toner is sufficiently melted when the image is fixed. In U.S. Pat. No. 4,549,803, a structure has been disclosed which employs the foregoing method and in which the fixing speed and the fixing temperature can be varied to be adaptable to the type of the image receiving sheet, for example, whether the sheet is plain paper or a transparent sheet for OHP (Over Head Projector). However, the foregoing structure suffers from a problem in that the structure of the image forming apparatus becomes too complicated to switch the fixing speed and the fixing temperature.
As another method of sufficiently melting toner when fixing is performed, a method has been suggested which uses so-called sharp melt toner having a low melting point, or having the melting viscosity which is rapidly lowered when the heated toner reaches the melting toner. However, simple use of the sharp melt toner is insufficient to form an excellent image. The reason for this is that a color image is formed by generally using toners in three colors, that is, cyan, yellow and magenta. Moreover, black toner is frequently used to remove the undercolor and to form a high contrast black characters. Therefore, the toner is, in the form of a multiplicity of layers, allowed to adhere to the surface of the image receiving sheet. Thus, the thickness of the toner image is enlarged as compared with a monochrome image. Therefore, by lowering the melting viscosity of the toner when fixing is performed is insufficient to attain the effect of smoothing the surface of the fixed image. In this case, the surface becomes irregular excessively and thus considerable irregular reflection takes place on the surface. Thus, the transparency is lowered and there arises a problem in that only a dark image can be formed. Since the thickness of the toner image is large, heat conductivity from the fixing means becomes insufficient or non-uniform when fixing is performed. As a result, toner cannot sufficiently be melted and thus a satisfactory effect of removing the interface between toner particles cannot be obtained. Therefore, color reproducibility deteriorates and there arises a problem in that a sharp color image cannot be formed. In general, the fixing means is a fixing means, for example, a known heat roller fixing means which is structured to heat and press a toner image to the image receiving sheet when fixing is performed. However, there arises a problem in that a so-called offset phenomenon takes place in which a portion of the toner is allowed to adhere to the fixing means in place of adhesion to the image receiving sheet. Moreover, in a case where the sharp melt toner is fixed to a recording medium (so-called rough paper), such as bond paper or regenerated paper, having coarse fibers and great irregularity on the surface of the paper, toner melted when fixing is performed and thus having a low viscosity is introduced into concave portions of the paper. Thus, there arises a problem in that convex portions in the regions which must be image portions and in which the surface of the paper must be covered with the toner are exposed in the image portions and thus the quality of the image deteriorates. What is worse, resin in the toner permeates fibers in the paper and thus luster becomes non-uniform along the fibers in the paper. As a result, there arises a problem of deterioration in the quality of the formed image.
To prevent the foregoing problems attributable to the thickness of the toner image, a structure in which the thickness of the toner image is reduced has been considered. However, the color development characteristic must be improved while reducing the thickness of the toner image because the image must have sufficiently high image density in order to obtain visibility of the image and practical image quality. In recent years, toner having significant coloring power has been investigated. Even if toner of the foregoing type is employed, it is preferable that a method of stacking color toners to express a required color be employed to cover the recording medium, such as paper, and obtain satisfactory color development characteristic while realizing high image resolution. Thus, the foregoing method involves a fact that the toner image on the image receiving sheet must have two or three layers. The toner having great coloring characteristic contains a coloring matter by about 6 wt % to 40 wt % which is about two to five times the quantity in the conventional toner in order to improve the coloring power. In general, pigment having excellent weathering resistance and heat resistance is generally employed as the coloring matter. However, the pigment cannot be dissolved by the binding resin which is one main components of the toner. The pigment exists in the toner in a state where it is dispersed in the binding resin. Therefore, if the quantity of the added pigment is too large, the quantity of the binding resin is correspondingly reduced and the dispersed pigment inhibits flow of the melted resin when the toner is fixed. Thus, there arises a problem of an unsatisfactory fixing ratio of the toner with respect to the recording medium, in particular, an unsatisfactory fixing ratio of the same with respect to a recording medium with which an anchoring effect which can be obtained because resin is introduced into small gaps between fibers of the paper cannot be expected, for example, a special sheet manufactured by forming synthetic resin into a sheet shape or a sheet for an OHP.
To prevent scattering of light caused by the fixed image, it is important to make sufficient smooth the surface of the fixed image and to prevent generation of an interface between toner particles of the fixed image, as described above. To realize this, another method has been suggested in which the recording medium is modified.
As a conventional structure of an image receiving sheet having an image receiving layer with which toner is fixed to the surface of the base sheet, a structure has been disclosed in U.S. Pat. No. No. 3,944,710 in which adhesivity between a transparent image receiving sheet and a multi-color image formed by toner is improved by forming a thin layer made of resin having a relatively low melting point on the surface of the image receiving sheet. However, there arises a problem in that the simple improvement of the adhesivity between the image receiving sheet and the image formed by the toner is insufficient to obtain satisfactory color development characteristic and transparency.
In U.S. Pat. Nos. 4,337,303 or 4,529,650, a structure has been disclosed in which the transparency of an image is improved by transferring a toner image to a transparent image receiving sheet having an image receiving layer on the surface thereof and simultaneously embedding the toner image in the image receiving layer which has been softened so as to fix the image. However, the above-mentioned structure capable of smoothing the surface of the fixed image by embedding the toner image in the image receiving layer cannot remove the interface between toner particles or between the toner and resin in the image receiving layer. In particular, the structure in which the toner is embedded in the image receiving layer encounters a problem in that an interface can easily be generated between toner and the resin in the image receiving layer. Therefore, there arises a problem in that satisfactory color development characteristic and transparency cannot be obtained.
The above-mentioned structure is formed such that the toner image is transferred and simultaneously it is embedded in the image receiving layer. Therefore, the image carrier for holding the toner image must be made of a material having a satisfactory heat resistance to prevent deterioration due to heat for softening the image receiving layer and excellent releasing characteristic to prevent adhesion of the image carrier and the softened image receiving layer. Therefore, there arises a problem in that selection of materials is limited and high-cost material must be employed.
In U.S. Pat. No. No. 5,378,576, a structure has been disclosed in which the surface of the fixed toner image is smoothed to prevent generation of pseudo outline due to irregular surface of the image and which has the steps of forming a resin layer on the surface of the image receiving sheet, the resin layer being composed of resin classified into a similar system in terms of the chemical structure to that of the resin in the toner and having a melting viscosity which is lower than that of the toner. The above-mentioned structure uses the resin, having the melting viscosity which is lower than that of the toner, to form the resin layer to discharge a portion of the resin layer when an image is fixed to the resin layer so as to smooth the boundary between the toner image portion and the non-image portion in order to prevent generation of the pseudo outline due to the irregular surface of the surface of the image.
However, the above-mentioned structure is formed such that the resins classified into similar systems in terms of the chemical structure are used as the resin forming the toner and that forming the resin layer so as to improve the affinity and the compatibility of the resins which are realized when they are melted. In order to prevent generation of an interface between the toner and the resin forming the resin layer, only consideration of the state where the resin forming the toner and the resin forming the resin layer are melted, that is, the fixed state is insufficient to prevent the problems in that satisfactory color development characteristic and transparency cannot be obtained.
Further, the foregoing structure must use toner having considerably low melting viscosity when the image is fixed to perfectly fuse the toner particles when gaps among the toner particles are attempted to be removed to prevent generation of the interface between the toner particles. Toner of a type having the foregoing thermal melting characteristic cannot stably be reserved. Moreover, also the realized mechanical strength is unsatisfactory. Therefore, there arises a problem in that melting and adhesion, that is so-called filming takes place in a press contact portion between a developing roller and a restraining blade disposed to be in contact with the developing roller and a press contact portion between the image carrier and the cleaning blade positioned in contact with the image carrier. Moreover, when the toner having a considerably low melting viscosity when fixing is performed is fixed to a recording medium, such as bond paper or regenerated paper, made of coarse fibers and involving great irregularity of the surface of the paper, the molten toner is introduced into the concave portions of the paper. Thus, regions of the paper to be covered with the toner to form the image are exposed in the image regions. As a result, there arises a problem in that the quality of the image deteriorates.
Further, as viewed from other aspect of the problem, color images have been required in the business field in recent years and thus a high quality color image is required to be formed on a rough paper, such as regenerated paper. To form a high quality image free from irregular luster and lacking of an image on the foregoing rough paper, use of toner having binding resin which has high viscosity when melted (specifically, having a high storage elastic modulus) has been considered. However, toner of the foregoing type raises a problem in that toner cannot sufficiently be made compatible when fixing is performed and thus an interface is generated between toner particles. Thus, the transparency and the color development characteristic are unsatisfactory. The foregoing problem becomes critical when an image is formed on an image receiving sheet for an OHP.
Since the rough paper has considerable irregularity on the surface thereof, the toner image cannot uniformly be transferred when the toner image is transferred to the surface of the paper. Thus, defective transference, such as non-uniform transference, takes place and thus there arises a problem in that a satisfactory image cannot be formed. The foregoing problem becomes critical with an apparatus of a type for forming a color image by forming a final image by stacking color images in a plurality of colors.
In recent years, the trend of wide use of color images in the business field arises a requirement for outputting color images at high speed, continuously and in a large quantity. To satisfy the foregoing requirements, durability and fluidity of toner are required to be improved. Specifically, materials and the quantity of inorganic or resin particles to be externally added to the surface of toner particles, that is, a so-called external additive are adequately adjusted. In particular, the quantity of the external additive has been enlarged.
When the quantity of the external additive is enlarged, the durability and fluidity of the toner can be improved and the transference efficiency can be raised. However, if the quantity of the external additive is enlarged, scattering of light and irregular reflection take place in the interface (a so-called grain boundary) between toner particles or an interface between toner and the image receiving layer or on the surface of the image. As a result, the light transparency deteriorates and the color development characteristic and the transparency deteriorate, thus causing a problem to arise in that a projected image is blackened on the image receiving sheet for an OHP. Therefore, there arise a problem in that an image having satisfactory color development characteristic and transparency cannot easily be obtained on luster paper and OHP film.
Moreover, an electrophotographic printer has been required to have further raised printing speed in order to reduce the size, save energy and to have performance superior to that of the ink jet printer. In view of the foregoing, the fixing means for heating and melting toner to fix the image on the recording paper must be able to fix the image while requiring smaller heating value. However, since a luster image can generally be formed only when the toner is sufficiently melted by the fixing means to make the surface to be smooth, a large heating value is required to fix the image. If the surface of the image is attempted to be made smooth with a small heating value, resin having a considerably low softening point must be used to form the toner or the image receiving layer of the image receiving sheet. Moreover, since the offset resistance and blocking resistance must be considered in addition to the smoothness, the thermal characteristic of the resin must be designed in a complicated manner.
Moreover, consideration must be performed to realize adequate winding of the sheet and conveyance easiness. Therefore, the image forming apparatus required to form a high quality image including the satisfactory luster property must use optimized toner and a fixing means as well as the image receiving sheet.
To satisfy the above-mentioned requirements, Japanese Patent Publication No. Hei. 2-263642 has disclosed a transparent laminate film comprising a transparent resin layer having a solubility parameter of 9.5 to 12.5 and a storage modulus (G') of 100 dyn/cm.sup.2 to 10000 dyn/cm.sup.2 at 160.degree. C. In accordance with the above-mentioned disclosure, resin having a storage modulus (G') greater than that of the binding resin forming the toner is employed as the transparent resin layer so that the light transmittance is improved.
According to Japanese Patent Publication No. Hei. 8-194394, the preferred range of the solubility parameter is 10 to 13 and that of the storage modulus (G'2) of the resin in the surface layer of the transfer paper with respect to the storage modulus (G'1) of the toner at 150.degree. C. is G'1-15 to G'1+150.
In Japanese Patent Publication No. Hei. 4-212168, a fact has been disclosed that the loss tangent of the resin in the coating layer is greater than that of toner or the resin for the toner.
However, the transparent resin layer disclosed in Japanese Patent Publication No. Hei. 2-263642 discusses the viscoelasticity realized when the fixing process is performed with only the storage modulus. In view point of the rheology, parameters, such as loss modulus (G") mainly indicating the characteristicas a viscous member and loss tangent indicating status change from the elastic deformation to the viscous deformation, must additionally be considered to perform advanced design and optimization. Similar facts are applied to the methods disclosed in Japanese Patent Publication No. Hei. 8-194394, Japanese Patent Publication No. Hei. 4-125567. In a case where resin having a storage modulus greater than that of the binding resin forming the toner is employed to form the transparent resin layer, manufactured toner has a low melting point and melting viscosity to perform the fixing process with the above-mentioned small heating value. Thus, the fluidity and the blocking resistance of the toner deteriorate, thus causing the amount of deformation of the toner particles to be enlarged considerably when the image has been fixed. In this case, a sharp image cannot be formed because dots and hair lines are deformed and spread. Moreover, the image forming apparatus involves a multiplicity of processes which are affected by filming and thus the apparatus must bear greater total load.
Although the value of the loss tangent of the toner or the resin for the toner and that of the resin in the coating layer have been discussed in Japanese Patent Publication No. Hei. 4-212168, the actual fixing characteristic is greatly affected by the relationship of the peak of the loss tangent indicating the status change of the resin with respect to the fixing temperature or the peak of the loss tangent of the toner because the loss tangent (G"/G') is the ratio of the loss modulus (G") and the storage modulus (G'). That is, the temperature at which the peak is attained is more important than the comparison at a certain temperature.
As described above, although a variety of structures of the image forming apparatus for outputting a color image and the image receiving sheet to be applied to the foregoing apparatus have been suggested, there arises a problem in that a high quality image having the color development characteristic and transparency equivalent to the silver salt photography cannot be obtained.